A white block can be found at the bottom of the screen that looks identical to a white block found in World 1-3 of Super Mario Bros 3. In Mario, holding 'down' on that block for about five seconds will drop you through it and into the background of the level.

Similarly, holding down on the white block in Celeste will drop you through it. From there, you can hop on the background art as if it were solid to climb up and reach the Crystal Heart.